1. Field of the Invention
The present invention generally related to a method for determining a real-time thickness of a single material layer, and more particularly to a method for determining an endpoint for a chemical-mechanical polishing (CMP) process.
2. Description of the Related Art
Chemical-mechanical polishing (CMP) is currently regarded as the only technology to provide global planarization in fabricating very-large scale integrated (VLSI) circuits, or even ultra-large scale integrated (ULSI) circuits. Essentially, the CMP is based on a principle similar to a "polishing wheel" in mechanical polishing. With the aid of a reagent, the rough surface of a wafer can then be smoothly ground by using this technology.
As the technologies of fabricating semiconductor devices enter the deep sub-micron regime, the CMP has become a conventional technology in fabricating ICs, in which an endpoint detection (EDP) method is the most critical factor in extending process window during the CMP process and overcoming stability problem in mass production.
To perform the endpoint detection for a dielectric layer, optical methods are the most popular ones, which measure the intensity of a reflected light as a function of time. Working together with the "Window Logic", turning-around points of slope on the curve can be identified, so that the endpoint of the CMP process can be determined. This method has advantages of real-time detection, no direct contact, and less noise induced.
The following paragraph describes the principles on which the endpoint detection method is based. Problems associated with this method are also described hereinafter.
Referring to FIG. 1, a light I.sub.inc is incident onto a dielectric layer 10. Being reflected from the top surface of the dielectric layer 10, and an interface between the dielectric layer 10 and an underlying substrate 12, the intensity of a reflected light I can be obtained from principles of optical interference: EQU I=I.sub.A +I.sub.B +2I.sub.A I.sub.B +L cos.phi. (1)
##EQU1##
and I is the intensity of a reflected light, t is the time, I.sub.A is the intensity of the first reflected light, I.sub.B is the intensity of the secondary reflected light, n is the refraction index of the material layer, d is the thickness of the material layer, .lambda..sub.0 is the wavelength of the incident light, .alpha..sub.ref is the refractive angle.
Eq. (1) shows that the intensity of the reflected light versus polishing time (I-t curve) comprises a cosine function as shown in FIG. 2. However, neither the thickness nor the endpoint of the process can be obtained from Eq. (1), unless a correlation between the intensity curve and the dielectric layer thickness was established. Consequently, it is very different to detect the endpoint of a CMP process on which applied to a single dielectric layer, for example, inter-layer dielectric (ILD) or shallow trench isolation (STI), since there is no strong signal difference coming from the interfaces of two different materials. Thus, the endpoint of the CMP process needs to be determined depending on the periodical variation of I-t curve.
For .phi.=m.pi.,m=0, 1, 2, . . . , the corresponding thickness d.sub.m can be obtained by ##EQU2##
the peaks and valleys on the I-t curve correspond to a certain thickness provides the initial thickness of the dielectric layer is know. Referring to and I-t curve obtained from polishing the whole thickness of a layer, these peaks and valleys form a set of characteristic points or indices, which can be used to determine the endpoint of the CMP process.
As shown in FIG. 2, a local minimum point 202 on the curve which is closest to the desired endpoint of a CMP process can be obtained, which is called the "analytical endpoint" 204 because it can be explicitly defined on the curve. This local minimum point 202 can be identified by "window logic", as it is the changing point of slope. After reaching the analytical endpoint 204, an over polishing step is further performed until reaching a required final thickness 206.
Thus, for a single dielectric layer (ILD) or a shallow trench isolation (STI) CMP process, the conventional endpoint detection technology identifies several local maximum/minimum points on the curve which have a thickness close to the desired thickness based on wavelength of the incident light.
However, there are still some problems for the above-mentioned principle to be applied practically. Although the peaks and valleys refers to a certain thickness are those identified with a slope of zero on the curve, it is not easy, however, to have an accurate calculation for these points because of the constraint on the speed of the polishing table. For example, sampling rate in a CMP process is generally only 1 sample/second due to a reasonable process parameter, which is not sufficient to accurately identify points with a zero slope. A more practical way to identify the peaks and valleys with zero slope is through the turning-around points on the curve by identifying changes between positive and negative slopes. FIG. 2 shows a conventional "window logic" method to determine the analytical endpoint, in which observing windows 208 are used to identify the turning-around points on the curve. This method has an advantage of simplicity, but associated accuracy and reoccurrence are not so good. To minimize errors that might occur due to noises and interference on the curve, consecutive three to four windows are normally required to define a turn-around point. The inaccuracy of this method therefore guarantees no successful results in detecting peaks and valleys on the curve.
Due to different material used, environmental and process variations, problems encountered so far toward mass production includes that the thickness determination may not reflect the true thickness accurately since signals varies upon polishing material. In addition, the CMP processes demanding a very short polishing time, especially those having a polishing time of less than a signal period, the conventional method is not applicable. Furthermore, the corresponding I-t curves for each wafer can be dirrerent in both amplitude and period, because of difference in wafer and environment. Usually, different endpoint thickness is required for different processes, so that the corresponding endpoint on the I-t curve is different. Therefore, feasible rules need to be established to solve these problems.
The reason behind the above-mentioned problems is that the conventional signal analysis technology fails to accurately identify the peaks and valleys on I-t curve. The conventional method fails to quickly and correctly identify the slope of a point on the curve, whether it is descending or ascending.